Fix numerous bugs in the 8568 UEC support

Actually, fixed a large bug in the UEC for *all* platforms.
How did this ever work?

uec_init() did not follow the spec for eth_init(), and returned
0 on success.  Switch it to return the link like tsec_init()
(and 0 on error)

The immap for the 8568 was defined based on MPC8568, rather than
CONFIG_MPC8568

CONFIG_QE was off

CONFIG_ETHPRIME was set to "Freescale GETH".  Now is "FSL UEC0"

Fixed a comment about the ranges for CONFIG_ETHPRIME if TSEC_ENET is
enabled

Signed-off-by: Andy Fleming <afleming@freescale.com>